HI @arminunruh
I entered the code (sub menus dropdown) but I don't understand why it doesn't work.
You could help me!

nav.primary .sub-menu li{
display: block;
position: relative;
padding-top: 10px!important;
opacity: 0;
transition: opacity 200ms ease;
-webkit-transition: opacity 200ms ease;
pointer-events: none;
position: absolute;
nav.primary li:hover .sub-menu{
opacity: 1;
pointer-events: auto;

the domain is https://www.proarta.ch/beta

the drop-down menu is up EXHIBITIONS